1. Field of the Invention
The present invention relates to liquid crystal display devices, and more particularly, to an In-Plane Switching (IPS) type liquid crystal display (LCD) device in which luminance, aperture, and storage capacitance are increased to improve picture quality, and a method for fabricating the same.
2. Discussion of the Related Art
In keeping pace with the development of an information oriented society, demands for display devices have gradually increased. To meet the demands, various flat display devices, such as Liquid Crystal Display (LCD), Plasma Display Panel (PDP), Electro Luminescent Display (ELP), and Vacuum Fluorescent Display (VFD), have been studied and are used in various apparatuses.
Of the various display devices, the LCD is replacing the Cathode Ray Tube (CRT) and is used mostly for mobile display devices due to its good picture quality, thinness, light weight, and low power consumption. Besides being used as mobile display devices as for example, monitors for notebook computers, the LCD has been developed as monitors for televisions to receive and display a broadcast signal, and monitors for desk-top computers.
The LCD includes a liquid crystal panel for displaying a picture and a driving part for applying a driving signal to the liquid crystal panel. The liquid crystal panel has opposing first and second glass substrates, and a liquid crystal layer between the first and second glass substrates.
The first glass substrate (also called a TFT array substrate) is provided with a plurality of gate lines arranged at regular intervals in one direction, a plurality of data lines arranged at regular intervals perpendicular to the gate lines, a plurality of pixel electrodes on a sub-pixel region defined by the gate lines and the data lines to form a matrix, and a plurality of thin film transistors that are switched in response to signals on the gate lines for transmission of signals on the data lines to the pixel electrodes.
On the second glass substrate (also called a color filter substrate), there are a black matrix layer for shielding light to parts excluding the pixel regions; R, B, G color filter layers for displaying colors; and a common electrode for displaying a picture. In the IPS type LCD, the common electrode is generally formed on the first glass substrate.
The LCD is driven based upon a principle of optical anisotropy and polarity of the liquid crystal. Because the liquid crystal is long and thin, molecules of the liquid crystals orient in one direction. If an electric field is applied to the liquid crystals, the orientation of the molecules can be controlled. Therefore, if the orientation of the molecules of the liquid crystals is controlled, in order to change the orientation of the molecules of the liquid crystals, light polarized by the optical anisotropy is modulated. Based upon the electric properties of the liquid crystal, there are a positive liquid crystal of which the dielectric anisotropy is positive (+) and a negative liquid crystal of which the dielectric anisotropy is negative (−). The positive liquid crystals have long axes of the liquid crystal molecules arranged in a direction of application of the field, and the negative liquid crystals have long axes of the liquid crystal molecules arranged in a direction perpendicular to the direction of application of the field.
FIG. 1 illustrates a disassembled perspective view of a part of a related art TN liquid crystal display device, including opposite lower substrate 1, an upper substrate 2, a liquid crystal layer 3 between the lower substrate 1 and the upper substrate 2.
The lower substrate 1 has a plurality of gate lines 4 arranged at regular intervals in one direction, and a plurality of data lines 5 arranged at regular intervals perpendicular to the gate lines 4, to define a plurality of sub-pixel ‘P’ regions. A pixel electrode 6 is formed in each of the sub-pixel regions ‘P’ at which the gate lines 4 and the data lines cross, and a thin film transistor ‘T’ is formed in each part at which the gate lines 4 and the data lines 4 cross. The upper substrate 2 has a black matrix layer 7 for shielding light to parts except the pixel regions ‘P’, and R, G, B color filter layers 8 for displaying colors, and a common electrode 9 for displaying a picture.
The thin film transistor ‘T’ has a gate electrode extending from the gate line 4, a gate insulating film (not shown) on an entire surface of the lower substrate 1, an active layer on the gate insulating film over the gate electrode, and a source electrode extending from the data line 5, and a drain electrode opposite to the source electrode. The pixel electrode 6 is formed of a transparent conductive metal, such as indium-tin-oxide (ITO) of which light transmittivity is comparably good.
The LCD can display a picture by orienting the liquid crystal layer 3 on the pixel electrode 6 by means of a signal applied through the thin film transistor ‘T’ and by controlling quantity of light transmitting the liquid crystal layer 3 depending on an extent of orientation of the liquid crystal layer 3. The LCD, driving the liquid crystal by field applied in up/down direction between the upper and lower substrate 2 and 1, has good transmissivity and aperture and prevents a liquid crystal cell from being broken by static electricity as the common electrode 9 of the upper substrate 2 serves as ground. However, the driving of the liquid crystals by the field applied in up/down direction has a disadvantage in that a view field angle characteristic is not good.
Consequently, to overcome the disadvantage, a new technology, i.e., the In-Plane Switching (IPS) type LCD has been suggested. A related art IPS type LCD will be described. FIG. 2 illustrates a section of a related art IPS type LCD.
Referring to FIG. 2, the related art IPS type LCD includes a pixel electrode 12 and a common electrode 13 formed on the same layer of a lower substrate 11, and a liquid crystal layer 14 between the lower substrate 11 and the upper substrate 15, wherein the liquid crystal layer is driven by a lateral field between the pixel electrode 12 and the common electrode 13 on the lower substrate 11. Thus, the IPS type LCD has the pixel electrode and the common electrode 13 on the same substrate.
FIGS. 3A and 3B illustrate a phase shift of liquid crystals at voltage turn on/off in an IPS mode.
FIG. 3A illustrates a turn off state in which no lateral field is formed between the pixel electrode 12 and the common electrode 13, wherefrom it can be noted that no phase shift of the liquid crystal layer 14 is taken place. For example, liquid crystal molecules in the liquid crystal layer 14 are tilted upward at 45° from a horizontal line between the pixel electrode 12 and the common electrode 13. FIG. 3B illustrates a turned on state in which a lateral field is formed between the pixel electrode 12 and the common electrode 13, wherefrom it can be noted that a phase of the liquid crystal layer 14 is shifted, such that the liquid crystal is rotated in a range of 45° in a horizontal direction in comparison to the turned off state in FIG. 3A.
As shown FIG. 4, in a case in which there is no lateral field voltage between the pixel electrode 12 and the common electrode 13, an orientation 16 of the liquid crystal molecules is the same with orientation of an initial alignment film (not shown). As shown in FIG. 4B, if the lateral field voltage is applied between the pixel electrode 12 and the common electrode 13, the liquid crystal molecules are oriented to correspond to a direction 17 of application of the field.
The IPS type LCD is advantageous in that it has a large viewing angle, a simple fabrication process, and a color shift following change of the view angle is small. The IPS type LCD is disadvantageous in that transmittivity of light and aperture are poor because the common electrode 13 and the pixel electrode 12 are on the same substrate. Moreover, with the IPS type LCD, the response time to a driving voltage needs improvement, and it is necessary to make cell gaps uniform because the misalignment margin of the cell gap is small.
The IPS type LCD will be described in more detail, with reference to the attached drawings. FIG. 5 illustrates a plan view of a related art IPS type LCD, and FIG. 6 illustrates sections across lines I–I′, and II–II′ in FIG. 5.
Referring to FIG. 5 and 6, a plurality of gate lines 61 and data lines 64_1, and 64_2 formed on a transparent lower substrate 60, to define a plurality of sub-pixel regions. Thin film transistors TFTs are formed on regions in which gate lines 61 and the data lines 64_1, and 64_2 cross.
The thin film transistor TFT has a gate electrode 61a at the gate line 61, a gate insulating film 62 on an entire surface of the lower substrate 60 inclusive of the gate electrode 61a, an active layer on the gate insulating film 62 over the gate electrode 61a, a source electrode 64a projected from the data line 64_1, and a drain electrode 64b opposite to the source electrode 64a. 
There are a common line 61b and common electrodes 61c on the same layer with the gate line 61, wherein the common line 61b is spaced from and parallel to the gate line 61, and a plurality of the common electrodes 61c are arranged in the sub-pixel regions in a direction parallel to the data line 64_1.
There is a protection film 65 on an entire surface inclusive of the data lines 64_1, and 64_2, having a contact hole 66 to expose a drain electrode 64b. The protection film 65 is a silicon nitride film.
There is a pixel electrode 67 on the protection film 65 in the sub-pixel region between and parallel to the common electrodes 61c connected to the drain electrode 64b of the thin film transistor through the contact hole 66. The pixel electrode 67 is a transparent conductive film.
The upper substrate 50, opposite to the lower substrate 60, has a color filter layer 52 at a part opposite to the sub-pixel region for displaying colors, and a black matrix layer 51 for isolating the color filter layers 52, and shielding a light. Reference numeral 68 denotes a storage electrode, forming a Storage On Common structure, wherein the pixel electrode overlaps a portion of the common line.
The black matrix layer 51 is formed on parts opposite to the gate line 61, the data line 64_1, and 64_2, a peripheral region inclusive of regions between the data lines 64_1, and 64_2 and the common electrodes 61c adjacent thereto, and the thin film transistors.
The liquid crystals between the common electrode 61c and the pixel electrode 67 are oriented in the same direction by the lateral field distributed between the common electrode 61c and the pixel electrode 67, to form one domain.
The foregoing related art IPS type LCD has the following problems.
The formation of the black matrix layer 51 on the data lines 64_1, and 64_2, and the common electrode 61c around the data lines 64_1, and 64_2, complicates the fabrication process because it is necessary to design the upper and lower substrates using a bonding margin required for bonding the upper and lower substrates into account. Also, luminance in the vicinity of the data lines 64_1, and 64_2 is reduced because the bonding margin of the upper and lower substrates 50 and 60 gradually increases.
That is, since it is required to form the black matrix layer 51, not only on the data lines 64_1, and 64_2, but also on regions between the data lines 64_1, and 64_2 and the common electrodes 61c adjacent thereto, the bonding margin is liable to cause an aperture loss and luminance drop.
The protection film of silicon nitride, which is comparatively thin in a range of 0.3 μm, may cause cross talk between the data line and the pixel electrode, and picture quality drop caused by a parasitic capacitance.